Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I understand the difference between best angle of climb $V_{x}$ and best rate of climb $V_{y}$. When I take off I would aim to maintain one speed or another depending on what I want.
But I don't understand why it's measured in airspeed, instead of pitch angle, which would seem more intuitive.
Because determining Vx and Vy has to do with the specific aircraft, specific power output of the engine, thrust from the propeller, etc, which is often a function of pressure altitude and airspeed.
Remember that Vx represents a situation where the aircraft has the MAXIMUM EXCESS THRUST meaning a situation where the difference between the maximum thrust available and and the thrust required to neutralize drag force and maintain straight and level flight is the greatest.
Images courtesy of BoldMethod
Vy on the other hand is the point where the aircraft has the MAXIMUM EXCESS POWER available to it, translating into the greatest change in altitude for a given period of time by simple conservation of energy considerations.
Images courtesy of BoldMethod
The airspeed where Vx occurs increases as the pressure altitude increases. Conversely Vy decreases as pressure altitude increases. This is due changes in power and thrust outputs with increasing pressure altitudes. Vx and Vy eventually equal each other at the aircraft’s absolute ceiling, since there is neither any more excess thrust or power available here for climbing.
A very simple practical reason is that Vx and Vy speeds can be easily monitored as the pilot is climbing away and controlled with the elevator. This useful information is in the POH to be used by people who are quite busy at the moment, there for it is expressed in a user friendly form.
Climb angle, as a matter of interest, could be studied for a variety of engine, prop, weight, flap setting, etc., but what the pilot needs on the spot is V at full throttle climb.
Let’s say you have an engine problem, or it’s hot and your performance is negatively effected. The angle of attack will be very different to get best climb, but the airspeed will be the same as if you have lots of power.
Pitch angle only corresponds to an angle or rate of climb at a particular excess thrust.
Consider the typical stall exercise: you pull the engine to idle, then apply more and more elevator to try to maintain altitude, thereby pitching up and simultaneously reducing airspeed. At one point, airspeed drops to the point where you lose lift, and you fall out of the sky, despite the fact that you are nose high.
Once you apply power and push the nose down a little, it again becomes easy to establish a climb to regain any altitude lost.
Heck, consider how you slow an airplane down in the air, by raising the nose without having excess thrust.
If rate of climb (expressed as either best angle or best rate) related only to pitch angle or angle of attack, then instead of slowing down, you'd start to climb and maintain that climb as long as the pitch angle is unchanged, no matter the power setting. That's clearly not happening, at least not for long, so there must be something else to determine whether the airplane actually climbs or descends. That factor, as already mentioned, is (excess) thrust.
Therefore, expressing Vx or Vy in terms of pitch angle would require taking into account the amount of excess thrust available. Accurately estimating that directly is non-trivial, at least without specific instrumentation, and would also likely require the pilot to memorize a whole table, rather than a single (or two) indicated airspeed(s). During a high-workload phase of flight, including take-off and initial climb-out, every little bit that adds to the mental workload of the pilot risks increasing the workload beyond the pilot's capabilities.
If instead you aim to maintain a given indicated airspeed, you will be at a point of the power curve where all the factors line up for that airspeed, regardless of the external conditions (pressure altitude, wind, etc.). If airspeed drops for any reason, you pitch down slightly to pick up more speed and maintain the climb; if airspeed rises, you pull the nose up instead to slow down (trading airspeed for climb angle). You're using the pitch angle to maintain an airspeed, and the only instrument you need to pay very close attention to is the airspeed indicator.
